1. Field of the Invention
The present invention is directed to an implantable energy storage system for a medical implant and includes a rechargeable storage for electrical energy and a unit for controlling the charging process. In addition, the present invention is directed to a process for operating such an energy storage system.
2. Description of Related Art
Energy storage systems of the afore-mentioned type and associated operating processes are disclosed in U.S. Pat. No. 5,411,537, U.S. Pat. No. 5,702,431, U.S. Pat. No. 5,713,939 and U.S. Pat. No. 6,154,677. Conventionally, implantable energy storage systems are recharged transcutaneously via an inductive path by means of an external charging device. The charging operation is conventionally controlled by way of a control unit which measures the charging current and the voltage of the storage and converts the measured values into corresponding control pulses for a switch provided in the charging circuit, with a suitable charging program being used.
When the energy storage is in operation, two undesirable operating states can occur, first, overcharging of the battery can occur if the charging process is not terminated at the proper time; which leads to gas evolution with subsequent destruction of the storage. Second, the storage voltage, when charging of the storage is not done on time, can drop to values which are below a minimum operating voltage which is necessary for defined operation or optionally for limited function of the implant which is to be supplied by the energy storage. In the latter case, under certain circumstances, the storage voltage can drop to such an extent that even supply of sufficient voltage to the implant-side electronics no longer ensures control of the charging process. Often, the controller includes a microprocessor system in which, in the case of undervoltage, wrong logic operations can occur or contents of volatile memories can be lost. Thus, in the case of undervoltage, in these systems proper charge control is no longer ensured, which can lead to the charging path being switched to high resistance by a microprocessor malfunction when entering the undervoltage range, whereby charging of the storage is permanently prevented.
Even if such a blockage of the charging path does not occur, a different problem can arise when the storage is being recharged. This results from the fact that, in power-saving circuits, Pierce oscillators are often used which are known to be difficult to start. To enable reliable transient oscillation of the oscillator, the power supply voltage must rise as quickly as possible from the undervoltage range. However, when the rechargeable storage is being charged, the voltage rises relatively slowly; this can lead to the oscillator not being able to start under certain circumstances, which prevents the microprocessor from being started up again.
A primary object of the present invention is to provide an implantable energy storage system for a medical implant, and a process for its operation, with safe operation being guaranteed at any time and safe recharging of the energy storage being possible even when the storage has been discharged, at least to such an extent that the storage voltage has dropped below the normal voltage range for the control unit.
This as well as other objects are achieved in accordance with the present invention by an implantable energy storage system for a medical implant and includes a rechargeable storage for electrical energy and a unit for controlling charging of the energy storage system, wherein a monitoring unit is provided which is independent of the control unit. The monitoring unit senses the storage voltage independently of the control unit and is made so as to assume control of the charging path when the sensed storage voltage lies outside of a predetermined range.
This as well as other objects are achieved in accordance with the present invention by a process for operating an implantable energy storage system for a medical implant including a rechargeable storage for electrical energy, wherein during normal operation the charging process is controlled by way of a control unit, wherein the storage voltage is sensed independently of the control unit by means of a monitoring unit which is independent of the control unit, and wherein the monitoring unit assumes control of the charging path when the sensed storage voltage lies outside a predetermined range. In this approach, in accordance with the present invention, it is advantageous that by providing a monitoring unit which is independent of the control unit, even when the control unit has failed, for example, due to very low storage voltage when the storage has been substantially discharged, proper charging control is possible, and especially the rechargeability of the storage can be ensured at any time by preventing blockage of the charging path due to malfunction of the control unit caused by undervoltage.
In a preferred embodiment of the invention, the control unit controls the charging process via a controllable resistance in the charging path, the resistance being made conductive by the monitoring unit when the sensed storage voltage falls below a predetermined first lower threshold value. Preferably, the control unit contains a microprocessor which, via a first switch, is supplied with power from the storage, the monitoring unit being chosen such that when the sensed storage voltage falls below a predetermined second lower threshold which preferably corresponds to the first lower threshold, the control unit cuts off the microprocessor system from the power supply by the storage by opening the first switch. In this way, the microprocessor is prevented from operating in an undervoltage range in which malfunctions can occur. In addition, in this way the power drain from the storage can be reduced; this increases the time interval to complete discharge of the storage and thus reduces the danger of complete discharge.
Preferably, the monitoring unit is made such that when the sensed storage voltage exceeds a predetermined first upper threshold, by closing the first switch it connects the microprocessor system to the power supply by the storage and transfers control of the first switch to the microprocessor. In this way, the power supply voltage is xe2x80x9csuddenlyxe2x80x9d turned on and thus a very steep rising edge is realized; this facilitates the stimulation of oscillations of an oscillator associated to the microprocessor. In addition, in this way, hysteresis can be set so that the microprocessor is restarted only when a fully sufficient storage voltage is reached. Furthermore, the monitoring unit is preferably designed such that, when the storage voltage exceeds a predetermined maximum value, overcharging of the storage independently of the control system is prevented by the monitoring unit switching the charging path such that charging current is prevented from continuing to flow to the storage.
A second preferred embodiment includes a bypass mechanism which is externally actuated to bypass the controllable resistor at least when the energy storage has been completely discharged. The bypass mechanism can be formed by a magnetically actuateable switch or by a diode which is poled in reverse direction. Even when the storage is completely discharged and even when the monitoring means can no longer operate, the bypass mechanism ensures that the storage can again be supplied with charging current.
These and further objects, features and advantages of the present invention will become apparent from the following description when taken in connection with the accompanying drawing which, for purposes of illustration only, shows an embodiment in accordance with the present invention.